I am a Vodacom customer, however one of your subsidiaries keeps harassing me with calls trying to sell me your product that I am already part of

I am a Vodacom customer, however one of your subsidiaries keeps harassing me with calls trying to sell me your product that I am already part of. I am writing exams and i keep getting interruptions and blocking that number does not work as it keeps changing. Please remove me off the subsidiaries contact list.